1. Technical Field
The present invention relates to technology that uses acoustic energy transmitted through a fluid for cleaning or some other purpose, and more particularly to a method that uses the emission of light from the fluid when it is exposed to the acoustic energy (sonoluminesence) to monitor and control the cavitation in the fluid.
2. Background Information
Historically, many industrial cleaning processes have made use of aqueous chemical fluids to remove particles from objects. In many cases, adding acoustic energy in the frequency range of 1 KHz to 10 MHz into the process bath has been shown to improve the cleaning process by removing particles more completely and in a shorter period of time. Introduction of acoustic energy into plating baths has also been shown to enhance electroplating processes by improved mixing of the chemistry and boundary layer, resulting in fresh species being available at the surface of the object being plated. Additionally, acoustic energy has been shown to help keep the cathode and anode clean.
Acoustically enhanced processes are commonly described as either ultrasonic or megasonic according to the frequency range of the induced sound field. The acoustic energy is commonly generated by exciting a piezoelectric crystal with a sinusoidal AC voltage. The crystal changes dimension at a rate determined by the frequency of the AC voltage. These periodic dimensional changes are mechanical vibrations, the energy from which is coupled into the process fluid through a resonator plate, thus creating an acoustic energy field. The crystal is typically tightly bonded to a transmitting member called a resonator that comes in contact with the fluid.
Ultrasonic cleaning is most appropriate for strong, heat tolerant substrate materials requiring cleaning of objects with moderately complex surface topologies. The ultrasonic frequency range is also well suited for removal of comparatively large particles from these chemically tolerant surfaces. Megasonic cleaning is appropriate for objects with heat and chemical sensitive surfaces, requiring line of sight dependent cleaning. Megasonic cleaning is also the method of choice for cleaning when the particle size is below approximately 0.3 μm.
Acoustic cavitation is generally regarded as the principle mechanism for particle removal in the cleaning process. In an acoustic field, a bubble or cavity is created when the high pressure tears the fluid, creating a bubble or void. These bubbles or voids are called cavities.
These pressure oscillations produce bubbles which expand and contract with the peaks and valleys of the pressure waves. As the bubbles expand and contract, some of the gases which form the bubble are absorbed into the fluid during contraction (compression cycle) and diffuse back into the bubble on expansion (decompression cycle). When the bubble reaches a size that can no longer be sustained by the force of the surface tension of the fluid competing against the force of the pressure differential created by passing acoustic waves, the bubble implodes.
There are two types of acoustic cavitation: stable and transient. In stable cavitation, a stable cavity (or bubble) is mostly gas filled and grows very slowly over many acoustic cycles. The energy released with an implosion event of a stable cavity is much less than that of a transient cavity. In transient cavitation, a transient cavity (or bubble) contains argon gas but has very little or no other gases in it. It will grow to a large size in only a few cycles and releases a much larger amount of energy upon collapse.
Sonoluminescence (SL) is the light released when the bubble collapses or more precisely, implodes. The pressure and speed of the implosion raises the gas inside the bubble to sufficiently high temperatures to cause emission of photons. The light emission from sonoluminescence has a frequency of 200 nm to 600 nm and is generally characterized as being in the UV spectrum.
The sonoluminescence spectrum starts at the roughly the midpoint of the visible light spectrum and extends well into the UV range. It occurs principally due to the presence of naturally occurring argon which is dissolved in the water. It is argon which is the major component in the plasma that is critical to the photon emission at the time of implosion. As cavities oscillate in size, the mostly gaseous nitrogen and oxygen molecules move back and forth in and out of the bubble with each pressure cycle change. Argon however does not, and consequently the concentration of argon inside the bubble begins to dramatically rise above the naturally occurring level of about 1% found in the earth's atmosphere. Therefore, when the bubble collapses, the predominant gas in the bubble is argon, and it is argon (and the other Noble gases) which primarily exhibit the property of sonoluminescence. Historically, sonoluminescence has been associated with transient cavitation, and was not thought to occur in stable cavitation.
The cavitation threshold is the point at which cavitation becomes predominately transient and the cavities begin to collapse violently emitting a high level of energy in the form of photons. In the past, sensors that detect sound pressure, known as hydrophones, have been used to detect transient cavitation because transient cavitation implosion events emit sufficient sound energy above the intrinsic detection threshold of the sensor. However, stable cavitation implosions emit far less energy, and are therefore undetectable by hydrophones, for practical purposes.
Another way to measure cavitation is through the use of cavitation cells, such as those described in the published Patent Cooperation Treaty document WO 02/05465 A1. Cavitation cells are used to sense fluid cavitation output directly in the cleaning bath, and are valuable tools for gathering general information on how well a bath is working or how one cleaning bath compares to another.
However, cavitation cells suffer from several problems. For example, cavitation cells cannot be left in the process bath during operation because the probe needs to be in the energy field to make its measurements and the object being cleaned also needs to be in the field. Additionally, the cell geometry does not guarantee that the fluid properties inside the cell are the same as what is in the tank, and gas buildup inside the cell affects data reproducibility.
In the early 1990's, scientists performed tests on a single bubble suspended in a fluid and determined that the oscillating bubble did emit photons at every negative pressure cycle. This was called SBSL for Single Bubble Sonoluminescence. It was theorized and confirmed that a sound field of many bubbles may have the same light synchronized response to the negative pressure cycles as a single bubble. This phenomenon became known as MBSL (Multi-Bubble Sonoluminescence) and is the dominant condition found in the tanks of most acoustically enhanced cleaning systems. The MBSL photon emission spectrum has since been measured and found to be similar to that of SBSL for similar fluid properties and conditions.
A collapsing bubble in close proximity to the surface of a sensitive substrate can exert very high localized pressure and temperatures, causing structural damage to the substrate. Cavitation implosions have been shown to have the energy of 50-150 atmospheres of pressure and temperatures of 5,500 degrees Kelvin. Exposure to the energy released by cavitation implosion events is known to be the primary mechanism for the erosion damage to the surfaces of sensitive and finely structured devices. Cleaning processes have been developed which achieve reasonably high yield rates with acceptably low damage rates. However, these processes lack real-time feedback to enable closed loop control of conditions in the cleaning tank based on cavitation level.
Recent cleaning processes have been developed that have such a low power or high frequency that commercial suppliers of the cleaning equipment have advertised them as having no cavitation. What is needed is technology that can characterize these low power cleaning processes and use them to prevent or mitigate the damage caused by transient cavitation.